Iraqi Journal of Chemical

Design Criteria of an Activated Carbon Bed for Dechlorination of Water
Muna Y. Abdul – Ahad
Environmental Engineering Department - College of Engineering - University of Baghdad – Iraq

Abstract
Granular carbon can be used after conventional filtration of suspended matter or, as a combination of filtration adsorption medium. The choice of equipment depends on the severity of the organic removal problem, the availability of
existing equipment, and the desired improvement of adsorption condition.
Design calculations on dechlorination by granular - carbon filters considering the effects of flow rate, pH , contact time,
head loss and bed expansion in backwashing , particle size, and physical characteristics were considered assuming the
absence of bacteria or any organic interface .
KEY WORDS: Activated carbon, dechlorination, water purification.
This advantage compensates for the cost differential of
pulverized carbon and granular carbon applied on a single
- use basis.

Introduction
THEORY
Granular- activated carbon adsorption is a reliable and
effective means of removing most organic impurities
found in potable water supplies. Plant operations and
pilot column studies have shown carbon filtration to be
an effective process for removing detergents (Flentje,
1964), insecticides, (Robeck, 1965), viruses (Robeck,
1964), specific chemical pollutants, (Dostal, 1965), and
taste and odor pollutants (Flentje, 1964). These results
confirm postulations that carbon bed filtration would
remove a high percentage of undesirable organic
contaminants from water efficiently over a wide range of
impurity concentration conditions.

Filtration – Adsorption:

The utilization of granu1ar- carbon filtration is a
relatively simple and economical procedure. It is possible
to adopt existing plant filters for a combination filtration adsorption unit process with minimum alteration, by
filling them with granular carbon.

The capacity of granular activated carbon for removing
viruses has been studied by (Rebeck et al, 1964), who
found that in clear water poliovirus was removed much
more readily by beds of fresh carbon than by sand beds.
Activated carbon, even if its adsorption capacity was
exhausted, still removed slightly more virus than did the
beds of sand, however. Suitably abrasion- resistant
granular activated carbons can serve both as filter media
and adsorbent. (Smith and skeel, 1964), have reported
that granular carbon beds are serving such a dual role in
several locations (ELD, E.F., 1961) . Filters with fresh
carbon were placed in service along with similar sand
medium filters. The carbon beds were (24 in.) deep and
were tested for both adsorption and filtration at
conventional sand filter rates. The results are summarized
in (Table 1) (Smith and skeel, 1964).

Tests (Joyce, 1966) had shown that, in accordance with
adsorption theory, granular carbon in beds is more
efficient than pulverized carbon used in slurry form in
accordance with conventional water plant procedures.

For (60) Days the carbon filters reduced threshold odor
from (70 to 4). At the same time, they reduced turbidity
to less than (0.07 Jackson units), a performance
somewhat superior to that of the sand filters. Super

IJCPE Vol.9 No.4 (2008)

01 ppm) chlorine
would be reached . unless existing conditions
significantly retard the process. (Fig.01 ppm). A break-point of (0. as a combination filtration adsorption medium. specific chemical pollutants.
A rise in temperature and a lowering of (pH) favor
dechlorination. means of dechlorination are required. granular carbon medium in a
( 1 mgd) filter (700 cu ft ) on dechlorination service alone
could process (700 mi1 gal) of ( 4 ppm ) free .
1971) indicates the relationship of these factors as they
vary from (pH7) and 21°C.
As indicated in ( Fig .8ppm) to less than (
0.
Dechlorination by granular carbon is extremely effective
and reliable. B1ending of
the fresh carbon effluent with partially exhausted .L. will reduce
dechlorination efficiency somewhat. Geyerm.4-2.
as well as the granular carbon itself. Geyerm. suspended matter. and Oukun.
concentration of influent and type of carbon are shown in
(Fig. Also (Gulp. It is unlikely that a deliberate change in
pH or temperature favoring dechlorination alone would
be economically feasible. G. in regard to its color and to iron. Because the granular activated carbon acts
principally as a catalyst for the reduction of hypochlorous
acid to chloride ion.carbon
42
IJCPE Vol..
Usually two or more units are used in parallel down flow
operation. Geyerm. could
adversely affect dechlorination efficiency.They are
based on chlorine breakpoint of (0. 1956).
Chlorine:
Granular activated carbon has long been used for the
removal of residual chlorine from water. such as those of detergents.5 gpm
/sq ft and 2.chlorine
influent water before a breakpoint of (0. These data are theoretic
values determined with chlorine in distilled water. The start-ups of the units are staggered so that
exhaustion of each bed will be in sequence.
2Cl 2  C  2H 2 O  4HCl  CO2
Or
Example:
Under conditions of sand filter service ( that is 2. and free residual
chlorine was reduced from ( 1. As super
chlorination finds wider accepter in the public water
supply industry.Design criteria of an activated carbon bed for dechlorination of water
chlorination preceded the filtration. 1971). Long chain organic
molecules. as noted above. and Oukun. and
certain adsorbed organics. a reduction in particle size reflected in the
reduction of mesh size from (8x30) to ( 14x40) allows a
doubling of flow rate without a sacrifice in efficiency. Geyerm.
1974) gave the following equation. 2)(Fairm. A bed processing water containing
(2ppm) chlorine under similar conditions would last
about (6 years).
insecticides. the carbon filters continued to produce water
that.1971) .
(Magee. have little apparent
effect upon the dechlorination reaction. and
Oukun.
Variance in hydraulic loading. 1971).
Dechlorination will proceed concurrently with
adsorption of organic contaminates.chloride system in
great detail to postulate a relationship between flow rates. studied the carbon .
Cl 2  H 2 O  2 H   2Cl   O
The chemisorbed nascent oxygen decomposes in either
of the following two ways.5 ft bed depth) . manganese. concentration of influent and effluent chlorine. but many common
water impurities. and the desired
improvement of adsorption conditions.01 ppm) Cl2
and the absence of bacteria or any organic interference
were assumed. Mesh size (8 x 30) and flow
rate was (l gpm / ft3). such as phenol.
bed depth.1) (Fairm .4 (2008)
. the capacity of the carbon is
determined not by normal adsorption parameters but by
other considerations. The life of
the carbon in dechlorination service is extremely long.
chlorine.
C X O X  C  CO
C X O X  C  CO2
This takes place on the surface of the carbon.1) (Fairm.
Design criteria
Granular carbon can be used after conventional filtration
of suspended matter or. The choice of equipment depends on
the severity of the organic contaminants (detergents. and turbidity content was of a quality equivalent
to or better than that produced by sand filters.25ppm) . The effect of mesh size is pronounced.
C
B  bed depth( ft )
B
Log I 

CB
filtration rate( gpm / sq ft ) V
Tests (Fairm. Their results for flow rates. viruses. and
taste and odor pollutants) removal problem.After their odor removal capacity was
exhausted . the
availability of existing equipment. and Oukun.9 No. had
determined efficiency values for specific carbons
available to industrial and municipal treatment operations
in the United States.

5.
Particle size of the carbon.10gpm/sq ft) and bed height (5-20
ft).01ppm 2 gpm / ft 3
PH= 7 of water. This phenomenon has been explored by
many authors.01 of Cl2 as
given by (Fig. and bed depths
are normally (2. 1) and (Table 4). and physical characteristics of
coal.10 V
(d)Bed volume=
44gpm
4 gpm / ft
3
V  4 gpm / f t3
 10 ft 3 for 8  30 mesh size
43
IJCPE Vol. (Weber and Morris. If granular
activated carbon is to be effective in turbidity removal. A summary
(1) Applying equation
(2)
C
B  bed depth( ft )
B
log I 

CE
filtrationrate( gpm/ sq ft ) V
(a) Referring to (Fig.7 gpm / sq ft .based granular carbon are presented in (Figs.Muna Y.1ppm Cl2
log
1.
(b) Calculation of B using (Fig.9 No. effluent =
0. such as head loss
and backwash expansion. 4 & 5)
and (Table 2) (Magee.
influent concentration=
1ppm Cl2. The three
mesh size carbons were approximately the same in all
respects except particle size.1965) of the effect of
particle size. At the same time. there may be reasons to limit the bed
depth and flow rate parameters to remove effectively
turbidity and to backwash properly the filter. 1965). At shallow bed
depths. A direct linear relationship between contact time
and carbon bed-performance was found at the Nitro
faci1ity in full scale plant tests and concurrent small
co1oumn tests. 1965) summarizes the results. 1)
For 8x30 mesh size and 2 gpm / ft3
flow rate. including.00
4

.ft). Abdul – Ahad
effluent in effect prolongs the life of the bed before
reactivation or replacement of the individual beds is
necessary. containing three mesh size carbons.
were examined at bed depths (2. improving adsorption
performance. bed depth. 1965) shows the relationship of
contact time and performance. 1965).
of design criteria is given in (Table 4) (Weber and
Morris.
should be considered carefully as a design factor. Applying
log
log
CI
B

CE
V
1 ppm
B

0. if too
small a particle size were chosen.
the two carbon mesh sizes used
are 8x30 and 14x40. Backwash
expansion data. the smallest particle size demonstrates its rapid
adsorption rate. three sets of three
columns in series.5 ft). At the deeper bed depths and longer
contact periods.
Reduction of particle size for a given set of flow
conditions is recognized to be a means of increasing
adsorption rates and. head loss. & 7. When bed depths at given
flow
rates
are
reduced
to
a
contact time function (gpm / cu ft).
0.
Design calculations
Flow rates are usually (2. 1) . T = 21°C & a break
point = 0. thereby.Varying the combined values of
these two factors can be thought of as adjusting the
contact time of the
water and the granular carbon
beds. 5. however. it
must be hard enough to withstand vigorous backwash
agitation.
In a study(Weber and Morris. (Table 3) (Weber and
Morris. in addition to contact time.3) (Dostal et al.5-10 ft) . When the granular
carbon bed is functioning both as a turbidity removal and
adsorption unit. Length of filter run in an
adsorption -filtration bed would also be a problem. During these tests flow rate conditions
were (3. It should be dense enough to
expand during the backwash cycle and to settle quickly
for immediate resumption of filtration.4 (2008)
. and flow rates on the
performance of an ABS system.
:. the difference in performance
due to adsorption rate is perhaps significant. B = 4 for 8x30 mesh size
(c) Applying equation again
for the actual design
flow rate = 44 gpm.
Reduction in particle size to improve adsorption must be
consistent with other significant factors. (Fig. the performance is
directly proportional to this function.5-5gpm/sq. 1965).